1. Field of the Invention
The present invention relates to fixing devices that are designed to change an approach guide, that guides a recording medium to a fixing nip, between different angles in conjunction with another operation such as changing between different fixing pressures, and to image forming apparatuses.
2. Description of the Related Art
A series of image forming operations performed in a typical image forming apparatus is as follows: an electrostatic latent image that is formed on an image carrier is developed and visualized as a toner image by a developer, the toner image is transferred onto a recording medium (e.g. a sheet of paper), the recording medium is conveyed to a fixing device, the toner image is fixed using heat and pressure that is applied by the fixing device, and the recording medium is discharged outside of the apparatus.
The fixing device includes a heating member and a pressing member that are rotatable and adjacent to each other. The pressing member is pressed against the heating member, producing a specific fixing pressure (nipping pressure) therebetween.
An exemplary situation will now be considered where an envelope, as a recording medium, is conveyed into such a fixing device. Envelopes are sheets of paper folded and pasted together at some portions thereof. When an envelope is conveyed into the fixing nip produced between the heating member and the pressing member, the envelope often becomes warped and wrinkled. To avoid this, when an envelope is conveyed into a fixing device, the fixing pressure is generally set to be as low as possible. Some known fixing devices include fixing-pressure-changing mechanisms designed to manually, or automatically, change the fixing pressure between different values that are provided for different types of recording media.
Such a fixing device includes an approach guide that guides the recording medium to the fixing nip. The approach guide is positioned the upstream in the recording medium conveyance direction of the fixing nip.
In a situation where a sheet of plain paper having a relatively small thickness is conveyed into the fixing device, the angle of the approach guide is preferably set so that a conveyance line formed at the fixing nip has a large curvature. This is because an increased area of the sheet of plain paper winds around the heating member, advantageously increasing fixability.
As mentioned above, envelopes are sheets of paper folded and pasted together at portions thereof. If the angle of the approach guide is set to a value that is intended for a sheet of plain paper and provides high fixability, the envelope curves due to the increased curvature. Consequently, even if the fixing pressure is reduced, the large difference in curvature produced between the front and back sides of the envelope, causes the envelope to easily wrinkle.